Inductive heating device, aerosol delivery system comprising an inductive heating device, and method of operating same

ABSTRACT

An inductive heating device ( 1 ) for heating an aerosol-forming substrate ( 20 ) comprising a susceptor ( 21 ) comprises: a device housing ( 10 ), a DC power source ( 11 ) for providing a DC supply voltage (V DC ) and a DC current (I DC ), a power supply electronics ( 13 ) comprising a DC/AC converter ( 132 ) comprising an LC load network ( 1323 ) comprising a series connection of a capacitor (C 2 ) and an inductor (L 2 ) having an ohmic resistance (R Coil ), a cavity ( 14 ) in the device housing ( 10 ) for accommodating a portion of the aerosol-forming substrate ( 20 ) to inductively couple the inductor (L 2 ) to the susceptor ( 21 ). The power supply electronics ( 13 ) further comprises a microcontroller ( 131 ) programmed to determine from the DC supply voltage (V DC ) and from the DC current (I DC ) an apparent ohmic resistance (R a ), and from the apparent ohmic resistance (R a ) the temperature (T) of the susceptor ( 21 ). It is further programmed to monitor changes in the apparent ohmic resistance (R a ) and to detect a puff when a decrease of the apparent ohmic resistance (R a ) is determined which is indicative of a temperature decrease of the susceptor ( 21 ) during a user inhalation.

The present invention relates to an inductive heating device for heatingan aerosol-forming substrate. The present invention also relates to anaerosol-delivery system comprising such an inductive heating device. Thepresent invention further relates to a method of operating suchaerosol-delivery system.

From the prior art aerosol-delivery systems are known which comprise anaerosol-forming substrate, typically a tobacco containing plug. To heatthe tobacco plug up to a temperature at which it is capable of releasingvolatile components that can form an aerosol, a heating element such asa heating blade (typically made of metal) is inserted into the tobaccoplug. The temperature of the heating blade which is in direct contactwith the aerosol-forming substrate (the tobacco plug) is determined asbeing representative of the temperature of the aerosol-formingsubstrate. The temperature of the heating blade is calculated using theknown relationship between the ohmic resistance of the heating blade andthe temperature of the heating blade. Therefore, during heating, bymonitoring the ohmic resistance of the heating blade (e.g. throughvoltage and amperage measurements) the temperature of the heating bladecan be determined at any time during a smoking run. Due to thecapability to determine the temperature at any time during a smoking runit is also possible to determine when a puff is taken by the user duringa smoking run, as during a puff cool air flows over the resistivelyheated blade resulting in a temporary temperature drop of the bladewhich can be detected.

Other aerosol-delivery systems comprise an inductive heating devicerather than a heating blade. The inductive heating device comprises aninductor arranged in thermal proximity of the aerosol-forming substrate,and the aerosol-forming substrate comprises a susceptor. The alternatingmagnetic field of the inductor generates eddy currents and hysteresislosses in the susceptor, causing the susceptor to heat theaerosol-forming substrate up to a temperature at which it is capable ofreleasing volatile components that can form an aerosol. Since theheating of the susceptor is performed in a contactless manner, there isno direct way to measure the temperature of the aerosol-formingsubstrate. For that reason, it is also difficult to determine when apuff is taken by the user during the smoking run.

However, it would also be desirable to be able to determine when a puffis taken during a smoking run when the aerosol-forming substrate isinductively heated. Thus, there is need for an inductive heating devicefor heating an aerosol-forming substrate allowing for determination whena puff is taken. There is also need of an aerosol-delivery systemcomprising temperature measurement of the aerosol-forming substrate.

The invention suggests an inductive heating device for heating anaerosol-forming substrate comprising a susceptor. The inductive heatingdevice according to the invention comprises:

-   -   a device housing    -   a DC power source for in operation providing a DC supply voltage        and a DC current,    -   a power supply electronics configured to operate at high        frequency, the power supply electronics comprising a DC/AC        converter connected to the DC power source, the DC/AC converter        comprising an LC load network configured to operate at low ohmic        load, wherein the LC load network comprises a series connection        of a capacitor and an inductor having an ohmic resistance,    -   a cavity arranged in the device housing, the cavity having an        internal surface shaped to accommodate at least a portion of the        aerosol-forming substrate, the cavity being arranged such that        upon accommodation of the portion of the aerosol-forming        substrate in the cavity the inductor of the LC load network is        inductively coupled to the susceptor of the aerosol-forming        substrate during operation.

The power supply electronics further comprises a microcontrollerprogrammed to in operation determine from the DC supply voltage of theDC power source and from the DC current drawn from the DC power sourcean apparent ohmic resistance, further programmed to in operationdetermine from the apparent ohmic resistance the temperature of thesusceptor of the aerosol-forming substrate. The mircrocontroller isfurther programmed to monitor changes in the apparent ohmic resistanceand to detect a puff when a decrease of the apparent ohmic resistance isdetermined which is indicative of a temperature decrease of thesusceptor during a user inhalation.

The aerosol-forming substrate is preferably a substrate capable ofreleasing volatile compounds that can form an aerosol. The volatilecompounds are released by heating the aerosol-forming substrate. Theaerosol-forming substrate may be solid or liquid or comprise both solidand liquid components. In a preferred embodiment, the aerosol-formingsubstrate is solid.

The aerosol-forming substrate may comprise nicotine. The nicotinecontaining aerosol-forming substrate may be a nicotine salt matrix. Theaerosol-forming substrate may comprise plant-based material. Theaerosol-forming substrate may comprise tobacco, and preferably thetobacco containing material contains volatile tobacco flavor compounds,which are released from the aerosol-forming substrate upon heating.

The aerosol-forming substrate may comprise homogenized tobacco material.Homogenized tobacco material may be formed by agglomerating particulatetobacco. Where present, the homogenized tobacco material may have anaerosol-former content of equal to or greater than 5% on a dry weightbasis, and preferably between greater than 5% and 30% by weight on a dryweight basis.

The aerosol-forming substrate may alternatively comprise anon-tobacco-containing material. The aerosol-forming substrate maycomprise homogenized plant-based material.

The aerosol-forming substrate may comprise at least one aerosol-former.The aerosol-former may be any suitable known compound or mixture ofcompounds that, in use, facilitates formation of a dense and stableaerosol and that is substantially resistant to thermal degradation atthe operating temperature of the aerosol-generating device. Suitableaerosol-formers are well known in the art and include, but are notlimited to: polyhydric alcohols, such as triethylene glycol,1,3-butanediol and glycerine; esters of polyhydric alcohols, such asglycerol mono-, di- or triacetate; and aliphatic esters of mono-, di- orpolycarboxylic acids, such as dimethyl dodecanedioate and dimethyltetradecanedioate. Particularly preferred aerosol formers are polyhydricalcohols or mixtures thereof, such as triethylene glycol, 1,3-butanedioland, most preferred, glycerine. The aerosol-forming substrate maycomprise other additives and ingredients, such as flavorants. Theaerosol-forming substrate preferably comprises nicotine and at least oneaerosol-former. In a particularly preferred embodiment, theaerosol-former is glycerine.

The DC power source generally may comprise any suitable DC power sourcecomprising in particular a power supply unit to be connected to themains, one or more single-use batteries, rechargeable batteries, or anyother suitable DC power source capable of providing the required DCsupply voltage and the required DC supply amperage. In one embodiment,the DC supply voltage of the DC power source is in the range of about2.5 Volts to about 4.5 Volts and the DC supply amperage is in the rangeof about 2.5 to about 5 Amperes (corresponding to a DC supply power inthe range of about 6.25 Watts and about 22.5 Watts). Preferably, the DCpower source comprises rechargeable batteries. Such batteries aregenerally available and have an acceptable overall volume of betweenapproximately 1.2-3.5 cubic centimeters. Such batteries may have asubstantially cylindrical or rectangular solid shape. Also, the DC powersource may comprise a DC feed choke.

As a general rule, whenever the term “about” is used in connection witha particular value throughout this application this is to be understoodsuch that the value following the term “about” does not have to beexactly the particular value due to technical considerations. However,the term “about” used in connection with a particular value is always tobe understood to include and also to explicitly disclose the particularvalue following the term “about”.

The power supply electronics is configured to operate at high frequency.For the purpose of this application, the term “high frequency” is to beunderstood to denote a frequency ranging from about 1 Megahertz (MHz) toabout 30 Megahertz (MHz), in particular from about 1 Megahertz (MHz) toabout 10 MHz (including the range of 1 MHz to 10 MHz), and even moreparticularly from about 5 Megahertz (MHz) to about 7 Megahertz (MHz)(including the range of 5 MHz to 7 MHz).

The power supply electronics comprises a DC/AC converter (which may beembodied as a DC/AC inverter) connected to the DC power source.

The LC load network of the DC/AC converter is configured to operate atlow ohmic load. The term “low ohmic load” is to be understood to denotean ohmic load smaller than about 2 Ohms. The LC load network comprises ashunt capacitor, and a series connection of a capacitor and an inductorhaving an ohmic resistance. This ohmic resistance of the inductor istypically a few tenths of an Ohm. In operation, the ohmic resistance ofthe susceptor adds to the ohmic resistance of the inductor and should behigher than the ohmic resistance of the inductor, since the suppliedelectrical power should be converted to heat in the susceptor to an ashigh extent as possible in order to increase efficiency of the poweramplifier and to allow transfer of as much heat as possible from thesusceptor to the rest of the aerosol-forming substrate to effectivelyproduce the aerosol.

A susceptor is a conductor which is capable of being inductively heated.“Thermal proximity” means that the susceptor is positioned relative tothe rest of the aerosol-forming substrate such that an adequate amountof heat is transferred from the susceptor to the rest of theaerosol-forming substrate to produce the aerosol.

Since the susceptor is not only magnetically permeable but alsoelectrically conductive (it is a conductor, see above), a current knownas eddy current is produced in the susceptor and flows in the susceptoraccording to Ohm's law. The susceptor should have low electricalresistivity ρ to increase Joule heat dissipation. In addition, thefrequency of the alternating eddy current has to be considered becauseof the skin effect (more than 98% of the electrical current flow withina layer four times the skin depth 6 from the outer surface of theconductor). Taking this into account the ohmic resistance R_(S) of thesusceptor is calculated from the equation

R _(S)=√{square root over (2πfμ ₀μ_(r))}

wherein

-   f denotes the frequency of the alternating eddy current-   μ₀ denotes the magnetic permeability of free space-   μ_(r) denotes the relative magnetic permeability of the material of    the susceptor, and-   ρ denotes the electrical resistivity of the material of the    susceptor.

The power loss P_(e) generated by the eddy current is calculated by theformula

P _(e) =I ² ·R _(S)

wherein

-   I denotes the amperage (rms) of the eddy current, and-   R_(S) denotes the electrical (ohmic) resistance of the susceptor    (see above)

From this equation for P_(e) and from the calculation of R_(S) it can beseen that for a material having a known relative magnetic permeabilityμ_(r) and a given electrical resistivity ρ it is evident that the powerloss P_(e) generated by the eddy current (through conversion to heat)increases with increasing frequency and increasing amperage (rms). Onthe other hand, the frequency of the alternating eddy current (andcorrespondingly of the alternating magnetic field inducing the eddycurrent in the susceptor) cannot be arbitrarily increased, since theskin depth 6 decreases as the frequency of the eddy current (or of thealternating magnetic field inducing the eddy current in the susceptor)increases, so that above a certain cut-off frequency no eddy currentscan be generated in the susceptor anymore since the skin depth is toosmall to allow eddy currents to be generated. Increasing the amperage(rms) requires an alternating magnetic field having a high magnetic fluxdensity and thus requires voluminous induction sources (inductors).

In addition, heat is produced in the susceptor through the heatingmechanism associated with hysteresis. The power loss generated byhysteresis is calculated from the equation

P _(H) =V·W _(H) ·f

wherein

-   V denotes the volume of the susceptor-   W_(H) denotes the work required to magnetize the susceptor along a    closed hysteresis loop in the B-H diagram, and-   f denotes the frequency of the alternating magnetic field.

The work W_(H) required to magnetize the susceptor along a closedhysteresis loop can also be expressed as

W _(H) =

H·dB

The maximum possible amount of W_(H) depends on material properties ofthe susceptor (saturation remanence B_(R), coercivity H_(C)), and theactual amount of W_(H) depends on the actual magnetization B-H loopinduced in the susceptor by the alternating magnetic field, and thisactual magnetization B-H loop depends on the magnitude of the magneticexcitation.

There is a third mechanism generating heat (power loss) in thesusceptor. This heat generation is caused by dynamic losses of themagnetic domains in the magnetically permeable susceptor material whenthe susceptor is subjected to an alternating external magnetic field,and these dynamic losses also generally increase as the frequency of thealternating magnetic field increases.

To be able to generate the heat in the susceptor in accordance with theafore-described mechanisms (mainly through eddy current losses andhysteresis losses), a cavity is arranged in the device housing. Thecavity has an internal surface shaped to accommodate at least a portionof the aerosol-forming substrate. The cavity is arranged such that uponaccommodation of the portion of the aerosol-forming substrate in thecavity the inductor of the LC load network is inductively coupled to thesusceptor of the aerosol-forming substrate during operation. This means,that the inductor of the LC load network is used to heat the susceptorthrough magnetic induction. This eliminates the need for additionalcomponents such as matching networks for matching the output impedanceof the Class-E power amplifier to the load, thus allowing to furtherminimize the size of the power supply electronics.

Overall, the inductive heating device according to the inventionprovides for a small and easy to handle, efficient, clean and robustheating device due to the contactless heating of the substrate. Forsusceptors forming low ohmic loads as specified above while having anohmic resistance significantly higher than the ohmic resistance of theinductor of the LC load network, it is thus possible to reachtemperatures of the susceptor in the range of 300-400 degrees Celsius infive seconds only or in a time interval which is even less than fiveseconds, while at the same time the temperature of the inductor is low(due to a vast majority of the power being converted to heat in thesusceptor).

As mentioned already, in accordance with one aspect of the inductiveheating device according to the invention the device is configured forheating an aerosol-forming substrate of a smoking article. Thiscomprises in particular, that power is provided to the susceptor withinthe aerosol-forming substrate such that the aerosol-forming substrate isheated to an average temperature of between 200-240 degrees Celsius.Even more preferably, the device is configured for heating atobacco-laden solid aerosol-forming substrate of a smoking article.

As the aerosol-forming substrate heats up, it is desirable to controlthe temperature thereof. This is not easy to achieve since heating ofthe aerosol-forming substrate is performed by a contactless (inductive)heating of the susceptor (mainly through hysteresis losses and eddycurrent losses, as describe above), whereas in prior art resistiveheating devices temperature control has been achieved by measuring thevoltage and current at the resistive heating element due to the lineardependency of the temperature of the resistive heating element and theohmic resistance of the heating element.

Surprisingly, in the inductive heating device according to the inventionthere is a strictly monotonic relationship between the temperature ofthe susceptor and the apparent ohmic resistance determined from the DCsupply voltage of the DC power source and from the DC current drawn fromthe DC power source. This strictly monotonic relationship allows for anunambiguous determination of the respective temperature of the susceptorfrom the respective apparent ohmic resistance in the (contactless)inductive heating device according to the invention, as each singlevalue of the apparent ohmic resistance is representative of only onesingle value of the temperature, there is no ambiguity in therelationship. This does not mean that the relationship of thetemperature of the susceptor and the apparent ohmic resistance isnecessarily linear, however, the relationship has to be strictlymonotonic to avoid any ambiguous allocation of one apparent ohmicresistance to more than one temperature. The strictly monotonicrelationship of the temperature of the susceptor and the apparent ohmicresistance thus allows for the determination and control of thetemperature of the susceptor and thus of the aerosol-forming substrate.As will be discussed in more detail below, in case the DC/AC convertercomprises a class-E amplifier the relationship between the temperatureof the susceptor and the apparent ohmic resistance is linear at leastfor the temperature range of interest.

Puff determination can be performed without the need of an additionalpuff sensor. This is due to the fact that once the user takes a puffthus drawing air past the aerosol-forming substrate, this leads to atemperature decrease of the susceptor. This temperature decrease of thesusceptor leads to a corresponding decrease of the apparent ohmicresistance, and the magnitude of this temperature decrease (resulting ina corresponding decrease of the apparent ohmic resistance) indicatesthat a puff has been taken by the user.

In accordance with one aspect of the inductive heating device accordingto the invention, the microcontroller is programmed to detect a puffwhen the decrease of the apparent ohmic resistance corresponds to atemperature decrease of the susceptor (21) in the range of 10° C. to100° C., more specifically in the range of 20° C. to 70° C.

In accordance with a further aspect of the inductive heating deviceaccording to the invention, the microcontroller is further programmed toallow for the detection of puffs having a duration in the range of 0.5seconds to 4 seconds, more specifically in the range of 1 second to 3seconds, and even more specifically of about 2 seconds. This limits theduration of a detectable puff. While some users prefer to take puffshaving a short duration only other users prefer to take puffs having anextended duration. Once the puff is finished the temperature increasesagain until the next puff is taken by the user or until the temperaturereaches a desired operating temperature.

According to a further aspect of the inductive heating device accordingto the invention, the device further comprises a counter for countingthe puffs already taken from the same aerosol-forming substrate, and(optionally) an indicator for showing to the user the number of puffsalready taken from the same aerosol-forming substrate, or the number ofpuffs remaining to be taken from the same aerosol-forming substrate, orboth the number of puffs already taken and the number of puffs remainingto be taken from the same aerosol-forming substrate. It helpful for theuser to know the number of puffs already taken or the number of puffsremaining to be taken from the same aerosol-forming substrate, or both,as this may help in making sure that the user always enjoys the fullflavor when taking a puff, since the number of puffs that can be takenfrom the same aerosol-forming substrate and developing the full flavoris limited.

In accordance with a further aspect of the inductive heating deviceaccording to the invention, the microcontroller is further programmed toallow for a maximum number of puffs to be taken from the sameaerosol-forming substrate. The microcontroller is programmed to stop thesupply of DC power from the DC power source to the DC/AC converter whenthe counter has counted the maximum number of puffs taken from the sameaerosol-forming substrate. This constructional measure makes sure thatthe user always enjoys the full flavour when taking a puff, since thepossible number of puffs that can be taken by a user from the sameaerosol-forming substrate is limited by the device, so that it isimpossible for the user to take more than the maximum possible number ofpuffs from the same aerosol-forming substrate.

Turning back to the determination of the apparent ohmic resistance,determination of the apparent ohmic resistance from the DC supplyvoltage of the DC power source and the DC current drawn from the DCpower source comprises measurement of both the DC supply voltage and theDC current. However, in accordance with one aspect of the inductiveheating device according to the invention, the DC power source may be aDC battery, in particular a rechargeable DC battery, for providing aconstant DC supply voltage. This allows for recharging the batteries,preferably through a connection to the mains via a charging devicecomprising an AC/DC converter. In the case of supply of a constant DCsupply voltage, it is still possible and may be desirable to measure theDC supply voltage, however, such measurement of the DC supply voltage isnot mandatory then (as the DC supply voltage is constant). However, thepower supply electronics comprises a DC current sensor for measuring theDC current drawn from the DC battery, so that the apparent ohmicresistance (which is representative of the temperature of the susceptor)can be determined from the constant DC supply voltage (regardless ofwhether this constant DC supply voltage is measured or is determined tohave the constant value) and the measured DC current the apparent ohmicresistance. Generally, this aspect allows for the measurement of the DCcurrent only without the need to also measure the DC supply voltage.

As has been mentioned above, in certain instances it is possible torefrain from a measurement of the DC supply voltage, however, inaccordance with one aspect of the inductive heating device according tothe invention the power supply electronics comprises a DC voltage sensorfor measuring the DC supply voltage of the DC power source so thatdetermination of the actual value of the DC supply voltage can bemeasured in any event.

As has been discussed above, the inductive heating device according tothe invention allows for a control of the temperature. To achieve thisin a particularly advantageous manner, in accordance with a furtheraspect of the inductive heating device according to the invention themicrocontroller is further programmed to interrupt generation of ACpower by the DC/AC converter when the determined temperature of thesusceptor of the aerosol-forming substrate is equal to or exceeds apreset threshold temperature, and in accordance with this aspect themicrocontroller is programmed to resume generation of AC power when thedetermined temperature of the susceptor of the aerosol-forming substrateis below the preset threshold temperature again. The term “interruptgeneration of AC power” is intended to cover cases in which more or lessno AC power is generated as well as cases in which generation of ACpower is only reduced to maintain the threshold temperature.Advantageously, this threshold temperature is the targeted operatingtemperature which may be, in particular a temperature in the range of300° C. to 400° C., for example 350° C. The inductive heating deviceaccording to the invention heats the susceptor of the aerosol-formingsubstrate until the susceptor has reached the preset thresholdtemperature corresponding to a respective apparent ohmic resistance. Atthat time, a further supply of AC power by the DC/AC converter isinterrupted so that further heating of the susceptor is stopped and thesusceptor is allowed to cool down. Once the temperature of the susceptoris below the preset threshold temperature again, this is detected bydetermination of a corresponding apparent ohmic resistance. At thattime, generation of AC power is resumed in order to keep the temperatureas close as possible to the targeted operating temperature. This can beachieved, for example, by adjusting the duty cycle of the AC powersupplied to the LC load network. This is described, in principle, in WO2014/040988.

As has already been mentioned above, in accordance with one aspect ofthe inductive heating device according to the invention, the DC/ACconverter comprises a Class-E power amplifier comprising a transistorswitch, a transistor switch driver circuit, and the LC load networkconfigured to operate at low ohmic load, and the LC load networkadditionally comprises a shunt capacitor.

Class-E power amplifiers are generally known and are described indetail, for example, in the article “Class-E RF Power Amplifiers”,Nathan O. Sokal, published in the bimonthly magazine QEX, editionJanuary/February 2001, pages 9-20, of the American Radio Relay League(ARRL), Newington, Conn., U.S.A. Class-E power amplifiers areadvantageous as regards operation at high frequencies while at the sametime having a simple circuit structure comprising a minimum number ofcomponents (e.g. only one transistor switch needed, which isadvantageous over Class-D power amplifiers which comprise two transistorswitches that have to be controlled at high frequency in a manner so asto make sure that one of the two transistors has been switched off atthe time the other of the two transistors is switched on). In addition,Class-E power amplifiers are known for minimum power dissipation in theswitching transistor during the switching transitions. Preferably, theClass-E power amplifier is a single-ended first order Class-E poweramplifier having a single transistor switch only.

The transistor switch of the Class-E power amplifier can be any type oftransistor and may be embodied as a bipolar-junction transistor (BJT).More preferably, however, the transistor switch is embodied as a fieldeffect transistor (FET) such as a metal-oxide-semiconductor field effecttransistor (MOSFET) or a metal-semiconductor field effect transistor(MESFET).

According to a further aspect of the inductive heating device accordingto the invention, the inductor of the LC load network comprises ahelically wound cylindrical inductor coil which is positioned on oradjacent the internal surface of the cavity.

In accordance with another aspect of the inductive heating deviceaccording to the invention the class E power amplifier has an outputimpedance, and the power supply electronics further comprises a matchingnetwork for matching the output impedance of the class E power amplifierto the low ohmic load. This measure may be helpful to further increasepower losses in the low ohmic load leading to an increased generation ofheat in the low ohmic load. For example, the matching network maycomprise a small matching transformer.

In accordance with a further aspect of the inductive heating deviceaccording to the invention, the total volume of the power supplyelectronics is equal to or smaller than 2 cm³. This allows for anarrangement of the batteries, the power supply electronics and thecavity in a device housing having an overall small size which isconvenient and easy to handle.

According to a further aspect of the inductive heating device accordingto the invention, the inductor of the LC load network comprises ahelically wound cylindrical inductor coil which is positioned on oradjacent the internal surface of the cavity. Advantageously, theinductor coil has an oblong shape and defines an inner volume in therange of about 0.15 cm³ to about 1.10 cm³. For example, the innerdiameter of the helically wound cylindrical inductor coil may be betweenabout 5 mm and about 10 mm, and may preferably be about 7 mm, and thelength of the helically wound cylindrical inductor coil may be betweenabout 8 mm and about 14 mm. The diameter or the thickness of the coilwire may be between about 0.5 mm and about 1 mm, depending on whether acoil wire with a circular cross-section or a coil wire with a flatrectangular cross-section is used. The helically wound inductor coil ispositioned on or adjacent the internal surface of the cavity. Ahelically wound cylindrical inductor coil positioned on or adjacent theinternal surface of the cavity allows to further minimize the size ofthe device.

Yet a further aspect of the invention relates to an aerosol-deliverysystem comprising an inductive heating device as described above and anaerosol-forming substrate comprising a susceptor. At least a portion ofthe aerosol-forming substrate is to be accommodated in the cavity of theinductive heating device such that the inductor of the LC load networkof the DC/AC converter of the inductive heating device is inductivelycoupled to the susceptor of the aerosol-forming substrate duringoperation.

By way of example, the aerosol-forming substrate may be anaerosol-forming substrate of a smoking article. In particular, theaerosol-forming substrate may be a tobacco-laden solid aerosol-formingsubstrate which may be used in smoking articles (such as, for example,cigarettes).

According to one aspect of the aerosol-delivery system according to theinvention, the susceptor is made of stainless steel. For example,various grades of stainless steel can be used such as stainless steelgrade 430 (SS430) or stainless steel grade 410 (SS410), stainless steelgrade 420 (SS420) or stainless steel grade 440 (SS440). Other grades ofstainless steel can also be used. For example, the susceptor is a singlesusceptor element which may be embodied as a strip, a sheet, a wire or afoil, and these susceptor elements may have different cross-sectionalgeometries such as rectangular, circular, elliptical, or othergeometries.

In accordance with a particular aspect of the aerosol-delivery systemaccording to the invention, the susceptor may comprises a flat strip ofstainless steel, the flat strip of stainless steel having a length in arange of about 8 millimeters to about 15 millimeters, preferably alength of about 12 millimeters. The flat strip further may have a widthin a range of about 3 millimeters to about 6 millimeters, preferably awidth of about 4 millimeters or about 5 millimeters. The flat stripfurther may have a thickness in a range of about 20 micrometers to about50 micrometers, preferably a thickness in a range of about 20micrometers to about 40 micrometers, for example a thickness of about 25micrometers or about 35 micrometers. One very specific embodiment of thesusceptor may have a length of about 12 millimeters, a width of about 4millimeters and a thickness of about 50 micrometers, and may be made ofstainless steel grad 430 (SS430). Another very specific embodiment ofthe susceptor may have a length of about 12 millimeters, a width ofeither about 5 millimeters and a thickness of about 50 micrometers, andmay be made of stainless steel grade 420 (SS430). Alternatively, thesevery specific embodiments may also be made from stainless steel grade420 (SS420).

Yet another aspect of the invention relates to a method of operating anaerosol-delivery system as described above, and this method comprisesthe steps of:

-   -   determining from the DC supply voltage of the DC power source        and from the DC current drawn from the DC power source an        apparent ohmic resistance,    -   determining from the apparent ohmic resistance the temperature        of the susceptor of the aerosol-forming substrate,    -   monitoring changes in the apparent ohmic resistance, and    -   detecting a puff when a decrease of the apparent ohmic        resistance is determined which is indicative of a temperature        decrease of the susceptor during a user inhalation.

In accordance with one aspect of the method according to the inventionthe step of detecting a puff comprises detecting a puff when thedecrease of the apparent ohmic resistance corresponds to a temperaturedecrease of the susceptor in the range of 10° C. to 100° C., morespecifically in the range of 20° C. to 70° C.

In accordance with a further aspect of the method according to theinvention, the step of detecting a puff further comprises allowing forthe detection of puffs having a duration in the range of 0.5 seconds to4 seconds, more specifically in the range of 1 second to 3 seconds, andeven more specifically of about 2 seconds.

In accordance with yet a further aspect of the method according to theinvention, the method comprises the steps of counting the puffs alreadytaken from the same aerosol-forming substrate, and (optionally) showingto the user the number of puffs already taken from the sameaerosol-forming substrate, or the number of puffs remaining to be takenfrom the same aerosol-forming substrate, or both the number of puffsalready taken and the number of puffs remaining to be taken from thesame aerosol-forming substrate.

In accordance with still a further aspect of the method according to theinvention, the method comprises the step of allowing for a maximumnumber of puffs to be taken from the same aerosol-forming substrate, andstopping the supply of DC power from the DC power source to the DC/ACconverter when the counter has counted the maximum number of puffs takenfrom the same aerosol-forming substrate.

According to one aspect of the method according to the invention, the DCpower source is a DC battery, in particular a rechargeable DC battery,and provides a constant DC supply voltage. The DC current drawn from theDC battery is measured for determining from the constant DC supplyvoltage and the measured DC current the apparent ohmic resistance.

In accordance with yet another aspect of the method according to theinvention, the method further comprises the steps of:

-   -   interrupting the generation of AC power by the DC/AC converter        when the determined temperature of the susceptor of the        aerosol-forming substrate is equal to or exceeds a preset        threshold temperature, and    -   resuming generation of AC power when the determined temperature        of the susceptor of the aerosol-forming substrate is below the        preset threshold temperature again.

As the advantages of the method according to the invention andparticular aspects thereof have already been discussed above, they arenot reiterated here.

Further advantageous aspects of the invention will become apparent fromthe following description of embodiments with the aid of the drawings inwhich:

FIG. 1 shows the general heating principle underlying the inductiveheating device of the invention,

FIG. 2 shows a block diagram of an embodiment of the inductive heatingdevice and the aerosol-delivery system according to the invention,

FIG. 3 shows an embodiment of the aerosol-delivery system according tothe invention comprising an inductive heating device having essentialcomponents arranged in a device housing,

FIG. 4 shows an embodiment of essential components of the powerelectronics of the inductive heating device according to the invention(without matching network),

FIG. 5 shows an embodiment of the inductor of the LC load network inform of a helically wound cylindrical inductor coil having an oblongshape,

FIG. 6 shows a detail of the LC load network comprising the inductivityand ohmic resistance of the coil, and in addition shows the ohmicresistance of the load,

FIG. 7 shows two signals representing the DC current drawn from the DCpower source vis-a-vis the temperature of the susceptor from which it isevident when a puff is taken,

FIG. 8 shows the temperature of two susceptors vis-a-vis the DC supplyvoltage of the DC power source and the DC current drawn from the DCpower source, and

FIG. 9 shows an equivalent circuit of the power electronics of theinductive heating device.

In FIG. 1 the general heating principle underlying the instant inventionis schematically illustrated. Schematically shown in FIG. 1 are ahelically wound cylindrical inductor coil L2 having an oblong shape anddefining an inner volume in which there is arranged a portion or all ofan aerosol-forming substrate 20 of a smoking article 2, theaerosol-forming substrate comprising a susceptor 21. The smoking article2 comprising the aerosol-forming substrate 20 with the susceptor 21 isschematically represented in the enlarged cross-sectional detail shownseparately on the right hand side of FIG. 1. As mentioned already, theaerosol-forming substrate 20 of the smoking article 2 may be atobacco-laden solid substrate, however, without being limited thereto.

In addition, in FIG. 1 the magnetic field within the inner volume of theinductor coil L2 is indicated schematically by a number of magneticfield lines B_(L) at one specific moment in time, since the magneticfield generated by the alternating current i_(L2) flowing through theinductor coil L2 is an alternating magnetic field changing its polarityat the frequency of the alternating current i_(L2) which may be in therange of about 1 MHz to about 30 MHz (including the range of 1 MHz to 30MHz), and may in particular be in the range of about 1 MHz to about 10MHz (including the range of 1 MHz to 10 MHz, and may especially besmaller than 10 MHz), and very particularly the frequency may be in therange of about 5 MHz to about 7 MHz (including the range of 5 MHz to 7MHz. The two main mechanisms responsible for generating heat in thesusceptor 21, the power losses P_(e) caused by eddy currents (closedcircle representing the eddy currents) and the power losses P_(h) causedby hysteresis (closed hysteresis curve representing the hysteresis), arealso schematically indicated in FIG. 1. With respect to these mechanismsit is referred to the more detailed discussion of these mechanismsabove.

FIG. 3 shows an embodiment of an aerosol-delivery system according tothe invention comprising an inductive heating device 1 according to theinvention. The inductive heating device 1 comprises a device housing 10which can be made of plastic, and a DC power source 11 (see FIG. 2)comprising a rechargeable battery 110. Inductive heating device 1further comprises a docking port 12 comprising a pin 120 for docking theinductive heating device to a charging station or charging device forrecharging the rechargeable battery 110. Still further, inductiveheating device 1 comprises a power supply electronics 13 which isconfigured to operate at the desired frequency. Power supply electronics13 is electrically connected to the rechargeable battery 110 through asuitable electrical connection 130. And while the power supplyelectronics 13 comprises additional components which cannot all be seenin FIG. 3, it comprises in particular an LC load network (see FIG. 4)which in turn comprises an inductor L2, this being indicated by thedashed lines in FIG. 3. Inductor L2 is embedded in the device housing 10at the proximal end of device housing 10 to surround a cavity 14 whichis also arranged at the proximal end of the device housing 10. InductorL2 may comprise a helically wound cylindrical inductor coil having anoblong shape, as shown in FIG. 5. The helically wound cylindricalinductor coil L2 may have a radius r in the range of about 5 mm to about10 mm, and in particular the radius r may be about 7 mm. The length 1 ofthe helically wound cylindrical inductor coil may be in the range ofabout 8 mm to about 14 mm. The inner volume accordingly, may be in therange of about 0.15 cm³ to about 1.10 cm³. taken from a particularaerosol-forming substrate.

Returning to FIG. 3, the inductive heating device further comprises acounter 134 for counting the number of puffs already taken from aparticular aerosol-forming substrate which preferably is (but does nothave to be) an integral part of the power supply electronics 13, as wellas an indicator 100 arranged in the device housing (for example adisplay) for either indicating the number of puffs already taken from aparticular aerosol-forming substrate, or for indicating the number ofpuffs remaining to be taken from this aerosol-forming substrate, orboth. The tobacco-laden solid aerosol-forming substrate 20 comprisingsusceptor 21 is accommodated in cavity 14 at the proximal end of thedevice housing 10 such that during operation the inductor L2 (thehelically wound cylindrical inductor coil) is inductively coupled tosusceptor 21 of the tobacco-laden solid aerosol-forming substrate 20 ofsmoking article 2. A filter portion of the smoking article 2 may bearranged outside the cavity 14 of the inductive heating device 1 so thatduring operation the consumer may draw the aerosol through the filterportion 22. Once the smoking article is removed from the cavity 14, thecavity 14 can be easily cleaned since except for the open distal endthrough which the aerosol-forming substrate 20 of the smoking article 2is to be inserted the cavity is fully closed and surrounded by thoseinner walls of the plastic device housing 10 defining the cavity 14.

FIG. 2 shows a block diagram of an embodiment of the aerosol-deliverysystem comprising the inductive heating device 1 according to theinvention, however, with some optional aspects or components as will bediscussed below. Inductive heating device 1 together with theaerosol-forming substrate 20 comprising the susceptor 21 forms anembodiment of the aerosol-delivery system according to the invention.The block diagram shown in FIG. 2 is an illustration taking the mannerof operation into account. As can be seen, the inductive heating device1 comprises a DC power source 11 (in FIG. 3 comprising the rechargeablebattery 110), a microcontroller (microprocessor control unit) 131, aDC/AC converter 132 (embodied as a DC/AC inverter), a matching network133 for adaptation to the load, and the inductor L2. Microprocessorcontrol unit 131, DC/AC converter 132 and matching network 133 as wellas inductor L2 are all part of the power supply electronics 13 (see FIG.1). The DC supply voltage V_(DC) and the DC current I_(DC) drawn fromthe DC power source 11 are provided by feed-back channels to themicroprocessor control unit 131, preferably by measurement of both theDC supply voltage V_(DC) and the DC current I_(DC) drawn from the DCpower source 11 to control the further supply of AC power P_(AC) to theLC load network, and in particular to inductor L2. This aspect of theinductive heating device according to the invention will be explained inmore detail below. A matching network 133 may be provided for optimumadaptation to the load but is not mandatory and is not contained in theembodiment described in more detail in the following.

FIG. 4 shows some essential components of the power supply electronics13, more particularly of the DC/AC converter 132. As can be seen fromFIG. 4, the DC/AC converter comprises a Class-E power amplifiercomprising a transistor switch 1320 comprising a Field Effect Transistor(FET) 1321, for example a Metal-Oxide-Semiconductor Field EffectTransistor (MOSFET), a transistor switch supply circuit indicated by thearrow 1322 for supplying the switching signal (gate-source voltage) tothe FET 1321, and an LC load network 1323 comprising a shunt capacitorC1 and a series connection of a capacitor C2 and inductor L2. Inaddition, the DC power source 11 comprising a choke L1 is shown forsupplying a DC supply voltage V_(DC), with a DC current I_(DC) beingdrawn from the DC power source 11 during operation. Also shown in FIG. 4is the ohmic resistance R representing the total ohmic load 1324, whichis the sum of the ohmic resistance R_(Coil) of the inductor L2 and theohmic resistance R_(Load) of the susceptor 21, as this is shown in FIG.6.

Due to the very low number of components the volume of the power supplyelectronics 13 can be kept extremely small. For example, the volume ofthe power supply electronics may be equal or smaller than 2 cm³. Thisextremely small volume of the power supply electronics is possible dueto the inductor L2 of the LC load network 1323 being directly used asthe inductor for the inductive coupling to the susceptor ofaerosol-forming substrate 20, and this small volume allows for keepingthe overall dimensions of the entire inductive heating device 1 small.In case a separate inductor other than the inductor L2 is used for theinductive coupling to the susceptor 21, this would automaticallyincrease the volume of the power supply electronics, this volume beingalso increased if a matching network 133 is included in the power supplyelectronics.

While the general operating principle of the Class-E power amplifier isknown and described in detail in the already mentioned article “Class-ERF Power Amplifiers”, Nathan O. Sokal, published in the bimonthlymagazine QEX, edition January/February 2001, pages 9-20, of the AmericanRadio Relay League (ARRL), Newington, Conn., U.S.A., some generalprinciples will be explained in the following.

Let us assume that the transistor switch supply circuit 1322 supplies aswitching voltage (gate-source voltage of the FET) having a rectangularprofile to FET 1321. As long as FET 1321 is conducting (“on”-state), itdoes essentially constitute a short circuit (low resistance) and theentire current flows through choke L1 and FET 1321. As FET 1321 isnon-conducting (“off”-state), the entire current flows into the LC loadnetwork since FET 1321 essentially represents an open circuit (highresistance). Switching the transistor between these two states invertsthe supplied DC voltage and DC current into an AC voltage and ACcurrent.

For efficiently heating the susceptor 21, an as large as possible amountof the supplied DC power is to be transferred in the form of AC power toinductor L2 (helically wound cylindrical inductor coil) and subsequentlyto the susceptor of aerosol-forming substrate 20 which is inductivelycoupled to inductor 2. The power dissipated in the susceptor 21 (eddycurrent losses, hysteresis losses) generates heat in the susceptor 21,as described further above. Or to say it in other words, powerdissipation in FET 1321 has to be minimized while maximizing powerdissipation in susceptor 21.

The power dissipation in FET 1321 during one period of the ACvoltage/current is the product of the transistor voltage and current ateach point in time during that period of the alternatingvoltage/current, integrated over that period, and averaged over thatperiod. Since the FET 1231 hast to sustain high voltage during a part ofthat period and conduct high current during a part of that period, ithas to be avoided that high voltage and high current exist at the sametime, since this would lead to substantial power dissipation in FET1231. In the “on-” state of FET 1231, the transistor voltage is nearlyzero when high current is flowing through the FET 1231. In the “off-”state of FET 1231, the transistor voltage is high but the currentthrough FET 1231 is nearly zero.

The switching transitions unavoidably also extend over some fractions ofthe period. Nevertheless, a high voltage-current product representing ahigh power loss in FET 1231 can be avoided by the following additionalmeasures. Firstly, the rise of the transistor voltage is delayed untilafter the current through the transistor has reduced to zero. Secondly,the transistor voltage returns to zero before the current through thetransistor begins to rise. This is achieved by load network 1323comprising shunt capacitor C1 and the series connection of capacitor C2and inductor L2, this load network being the network between FET 1231and the load 1324. Thirdly, the transistor voltage at turn-on time ispractically zero (for a bipolar-junction transistor “BJT” it is thesaturation offset voltage V_(o)). The turning-on transistor does notdischarge the charged shunt capacitor C1, thus avoiding dissipating theshunt capacitor's stored energy. Fourthly, the slope of the transistorvoltage is zero at turn-on time. Then, the current injected into theturning-on transistor by the load network rises smoothly from zero at acontrolled moderate rate resulting in low power dissipation while thetransistor conductance is building up from zero during the turn-ontransition. As a result, the transistor voltage and current are neverhigh simultaneously. The voltage and current switching transitions aretime-displaced from each other.

For dimensioning the various components of the DC/AC converter 132 shownin FIG. 4, the following equations have to be considered, which aregenerally known and have been described in detail in the afore-mentionedarticle “Class-E RF Power Amplifiers”, Nathan O. Sokal, published in thebimonthly magazine QEX, edition January/February 2001, pages 9-20, ofthe American Radio Relay League (ARRL), Newington, Conn., U.S.A.

Let Q_(L) (quality factor of the LC load circuit) be a value which is inany event greater than 1.7879 but which is a value that can be chosen bythe designer (see the afore-mentioned article) let further P be theoutput power delivered to the resistance R, and let f be the frequency,then the various components are numerically calculated from thefollowing equations (V_(o) being zero for FETs, and being the saturationoffset voltage for BJTs, see above):

L2=Q _(L) ·R/2πf

R=((V _(CC) −V _(o))² /P)·0.576801·(1.0000086−0.414395/Q _(L)−0.557501/Q_(L) ²+0.205967/Q _(L) ³)

C1=(1/(34.2219·f·R))·(0.99866+0.91424/Q _(L)−1.03175/Q _(L)²)+0.6/(2πf)²·(L1)

C2=(1/2πfR)·(1/Q _(L)−0.104823)·(1.00121+(1.01468/Q_(L)−1.7879))−(0.2/((2πf)² ·L1)))

This allows for a rapid heating up of a susceptor having an ohmicresistance of R=0.6Ω to deliver approximately 7 W of power in 5-6seconds assuming that a current of approximately 3.4 A is availableusing a DC power source having a maximum output voltage of 2.8 V and amaximum output current of 3.4 A, a frequency of f=5 MHz (dutyratio=50%), an inductivity of inductor L2 of approximately 500 nH and anohmic resistance of the inductor L2 of R_(Coil)=0.1Ω, an inductivity L1of about 1 pH, and capacitances of 7 nF for capacitor C1 and of 2.2 nFfor capacitor C2. The effective ohmic resistance of R_(Coil) andR_(Load) is approximately 0.6Ω. An efficiency (Power dissipated in thesusceptor 21/maximum power of DC power source 11) of about 83.5% may beobtained which is very effective.

For operation, the smoking article 2 is inserted into the cavity 14 (seeFIG. 2) of the inductive heating device 1 such that the aerosol-formingsubstrate 20 comprising the susceptor 21 is inductively coupled toinductor 2 (e.g. the helically wound cylindrical coil). Susceptor 21 isthen heated for a few seconds as described above, and then the consumermay begin drawing the aerosol through the filter 22 (of course, thesmoking article does not necessarily have to comprise a filter 22).

The inductive heating device and the smoking articles can generally bedistributed separately or as a kit of parts. For example, it is possibleto distribute a so-called “starter kit” comprising the inductive heatingdevice as well as a plurality of smoking articles. Once the consumer haspurchased such starter kit, in the future the consumer may only purchasesmoking articles that can be used with this inductive heating device ofthe starter kit. The inductive heating device is easy to clean and incase of rechargeable batteries as the DC power source, theserechargeable batteries are easy to be recharged using a suitablecharging device that is to be connected to the docking port 12comprising pin 120 (or the inductive heating device is to be docked to acorresponding docking station of a charging device).

It has already mentioned above, that by determination of the apparentohmic resistance R_(a) from the DC supply voltage V_(DC) of the DC powersource 11 and from the DC current I_(DC) drawn from the DC power source11 it is possible to determine the temperature T of the susceptor 21.This is possible because surprisingly the relationship of thetemperature T of the susceptor 21 and the quotient of the DC supplyvoltage V_(DC) and DC current I_(DC) is strictly monotonic, and may evenbe practically linear for a Class-E amplifier. Such a strictly monotonicrelationship is shown in FIG. 8 by way of example. As already mentioned,the relationship does not mandatorily have to be linear, it only has tobe strictly monotonic so that for a given DC supply voltage V_(DC) thereis an unambiguous relationship between the respective DC current I_(DC)and the temperature T of the susceptor. Or in other words, there is anunambiguous relationship between an apparent ohmic resistance R_(a)(determined from the quotient of the DC supply voltage V_(DC) and the DCcurrent I_(DC) drawn from the DC power source) and the temperature T ofthe susceptor. This corresponds to an equivalent circuit shown in FIG. 9wherein R_(a) corresponds to a series connection formed by an ohmicresistance R_(CIRCUIT) (which is substantially smaller than the ohmicresistance of the susceptor) and a temperature dependent ohmicresistance R_(SUSCEPTOR) of the susceptor.

As mentioned already, in case of a Class-E amplifier this strictlymonotonic relationship between the apparent ohmic resistance R_(a) andthe temperature T of the susceptor is practically linear, at least forthe temperature range of interest (for example for the temperature rangebetween 100° C. and 400° C.)

If the relationship between the apparent ohmic resistance R_(a) and thetemperature T of a specific susceptor made of a specific material andhaving a specific geometry is known (for example, such relationship canbe determined through precise measurements in the laboratory for a largenumber of identical susceptors and subsequent averaging of theindividual measurements), this relationship between the apparent ohmicresistance R_(a) and the temperature T of this specific susceptor can beprogrammed into the microcontroller 131 (see FIG. 2) so that duringoperation of the aerosol-delivery system only the apparent ohmicresistance R_(a) has to be determined from the actual DC supply voltageV_(DC) (typically this is the constant battery voltage) and the actualDC current I_(DC) drawn from the DC power source 11. A large number ofsuch relationships between R_(a) and the temperature T can be programmedinto the microcontroller 131 for susceptors made of different materialsand having different geometries, so that during operation of theaerosol-forming device only the respective type of susceptor has to beidentified and then the corresponding relationship (already programmedin the microcontroller) can be used for the determination of thetemperature T of the respective type of susceptor actually used bydetermination of the actual DC supply voltage and the actural DC currentdrawn from the DC power source.

It is possible and may be preferred that both the DC supply voltageV_(DC) and the DC current I_(DC) drawn from the DC power source 11 canbe measured (this can be achieved with a suitable DC voltage sensor anda suitable DC current sensor which can be easily integrated in the smallcircuit without any relevant space consumption). However, in case of aDC power source of constant supply voltage V_(DC) a DC voltage sensorcan be dispensed with and only a DC current sensor is needed for themeasurement of the DC current I_(DC) drawn from the DC power source 11.

In FIG. 7 two signals are shown representing the DC current I_(DC) drawnfrom the DC power source 11 (upper signal) and the temperature T of thesusceptor 21 (lower signal) determined from the relationship between theapparent ohmic resistance R_(a) and the temperature T for this susceptor21 which is programmed in the microcontroller 131.

As can be seen, once the heating of the susceptor of the aerosol-formingsubstrate has started, the current I_(DC) is at a high level anddecreases as the temperature T of the susceptor of the aerosol-formingsubstrate increases (the increase in temperature of the susceptor leadsto an increase of R_(a) which in turn leads to a decrease of I_(DC)).

At different times during this heating process (in particular when theaerosol-forming substrate has reached a certain temperature), the usermay take a puff from the smoking article comprising the aerosol-formingsubstrate with the susceptor arranged therein. At that time, air drawnin during a puff of a duration D leads to a quick decrease ΔT of thetemperature of the aerosol-forming substrate 20 and of the susceptor 21.This temperature decrease ΔT leads to a decrease in the apparent ohmicresistance R_(a), and this in turn leads to an increase in the DCcurrent I_(DC) drawn from the DC power source 11. These points in timewhen the user takes a puff are indicated in FIG. 7 by the respectivearrows (except for the first puff where the duration D of the puff andthe decrease in temperature ΔT are indicated). Once the puff isfinished, air is no longer drawn in and the temperature of the susceptorincreases again (leading to a respective increase of the apparent ohmicresistance R_(a) and of the temperature T of the susceptor) and the DCcurrent I_(DC) decreases accordingly.

By way of example only, the puffs shown in FIG. 7 are taken every thirtyseconds and have a duration D of two seconds while each puff comprises avolume of fifty-five milliliters of air are drawn in, and thetemperature decrease ΔT of the susceptor 21 is, for example, about 40°C. Once a temperature decrease ΔT indicative of such puff is detected,the microcontroller 131 causes the counter 134 counting the puffs takenfrom the same aerosol-forming substrate to be increased by one and theindicator 100 indicating either the number of puffs taken from the sameaerosol-forming substrate or the number of puffs remaining to be takenfrom that aerosol-forming substrate, or both, to be increased/decreasedby one accordingly. Once the puff is finished and the temperature T ofthe susceptor 21 and the apparent ohmic resistance R_(a) increase againduring heating (as described above) the DC current I_(DC) drawn from theDC power source 11 decreases accordingly.

As can further be seen in FIG. 7, the DC/AC converter generates AC poweruntil the temperature of the susceptor 21 is equal to or exceeds apreset threshold temperature T_(th). Once the temperature of thesusceptor of the aerosol-forming substrate is equal to or exceeds thispreset threshold temperature I_(th) (e.g. a targeted operatingtemperature) the microcontroller 131 is programmed to interrupt furthergeneration of AC power by the DC/AC converter 132. It is then desired tomaintain the temperature T of the susceptor 21 at the targeted operatingtemperature. At the time the temperature T of the susceptor 21 is belowthe threshold temperature I_(th) again, the microcontroller 131 isprogrammed to resume generation of AC power again.

This can be achieved, for example, by adjusting the duty cycle of theswitching transistor. This is described in principle in WO 2014/040988.For example, during heating the DC/AC converter continuously generatesalternating current that heats the susceptor, and simultaneously the DCsupply voltage V_(DC) and the DC current I_(DC) are measured every 10milliseconds for a period of 1 millisecond. The apparent ohmicresistance R_(a) is determined (by the quotient of V_(DC) and I_(DC)),and as R_(a) reaches or exceeds a value R_(a) corresponding to thepreset threshold temperature T_(th) or to a temperature exceeding thepreset threshold temperature T_(th) the switching transistor 1231 (seeFIG. 4) is switched to a mode in which it generates pulses only every 10milliseconds for a duration of 1 millisecond (the duty cycle of theswitching transistor is only about 9% then). During this 1 millisecondOn-state (conductive state) of the switching transistor 1231 the valuesof the DC supply voltage V_(DC) and of the DC current I_(DC) aremeasured and the apparent ohmic resistance R_(a) is determined. As theapparent ohmic resistance R_(a) is representative of a temperature T ofthe susceptor 21 which is below the preset threshold temperature I_(th),the transistor is switched back to the mode mentioned above (so that theduty cycle of the switching transistor is more or less 100% again).

For example, the a susceptor 21 may have a length of about 12millimeters, a width of about 4 millimeters and a thickness of about 50micrometers, and may be made of stainless steel grad 430 (SS430). As analternative example, the susceptor may have a length of about 12millimeters, a width of either about 5 millimeters and a thickness ofabout 50 micrometers, and may be made of stainless steel grade 420(SS430). These susceptor may also be made from stainless steel grade 420(SS420).

Having described embodiments of the invention with the aid of thedrawings, it is clear that many changes and modifications areconceivable without departing from the general teaching underlying theinstant invention. Therefore, the scope of protection is not intended tobe limited to the specific embodiments, but rather is defined by theappended claims.

1. Inductive heating device for heating an aerosol-forming substratecomprising a susceptor, the inductive heating device comprising: adevice housing, a DC power source for in operation providing a DC supplyvoltage (V_(DC)) and a DC current (I_(DC)), a power supply electronicsconfigured to operate at high frequency, the power supply electronicscomprising a DC/AC converter connected to the DC power source, the DC/ACconverter comprising an LC load network configured to operate at lowohmic load, wherein the LC load network comprises a series connection ofa capacitor and an inductor having an ohmic resistance, a cavityarranged in the device housing, the cavity having an internal surfaceshaped to accommodate at least a portion of the aerosol-formingsubstrate, the cavity being arranged such that upon accommodation of theportion of the aerosol-forming substrate in the cavity the inductor ofthe LC load network is inductively coupled to the susceptor of theaerosol-forming substrate during operation, wherein the power supplyelectronics further comprises a microcontroller programmed to inoperation determine from the DC supply voltage of the DC power sourceand from the DC current drawn from the DC power source an apparent ohmicresistance, further programmed to in operation determine from theapparent ohmic resistance the temperature of the susceptor of theaerosol-forming substrate, and further programmed to monitor changes inthe apparent ohmic resistance and to detect a puff when a decrease ofthe apparent ohmic resistance is determined which is indicative of atemperature decrease of the susceptor during a user inhalation. 2.Inductive heating device according to claim 1, wherein themicrocontroller is programmed to detect a puff when the decrease of theapparent ohmic resistance corresponds to a temperature decrease of thesusceptor in the range of 10° C. to 100° C.
 3. Inductive heating deviceaccording to claim 1, wherein the microcontroller is further programmedto allow for the detection of puffs having a duration in the range of0.5 seconds to 4 seconds.
 4. Inductive heating device according to claim1, further comprising a counter for counting the puffs already takenfrom the same aerosol-forming substrate, and optionally an indicator forshowing to the user the number of puffs already taken from the sameaerosol-forming substrate, or the number of puffs remaining to be takenfrom the same aerosol-forming substrate, or both the number of puffsalready taken and the number of puffs remaining to be taken from thesame aerosol-forming substrate.
 5. Inductive heating device according toclaim 4, wherein the microcontroller is further programmed to allow fora maximum number of puffs to be taken from the same aerosol-formingsubstrate, and wherein the microcontroller is programmed to stop thesupply of DC power from the DC power source to the DC/AC converter whenthe counter has counted the maximum number of puffs taken from the sameaerosol-forming substrate.
 6. Inductive heating device according toclaim 1, wherein the device is configured for heating an aerosol-formingsubstrate of a smoking article.
 7. Inductive heating device according toclaim 1, wherein the DC power source is a DC battery, in particular arechargeable DC battery, for providing a constant DC supply voltage, andwherein the power supply electronics further comprises a DC currentsensor for measuring the DC current drawn from the DC battery fordetermining from the constant DC supply voltage and the measured DCcurrent the apparent ohmic resistance.
 8. Inductive heating deviceaccording to claim 1, wherein the power supply electronics furthercomprises a DC voltage sensor for measuring the DC supply voltage of theDC power source.
 9. Inductive heating device according to claim 1,wherein the microcontroller is further programmed to interruptgeneration of AC power by the DC/AC converter when the determinedtemperature of the susceptor of the aerosol-forming substrate is equalto or exceeds a preset threshold temperature, and wherein themicrocontroller is programmed to resume generation of AC power when thedetermined temperature of the susceptor of the aerosol-forming substrateis below the preset threshold temperature again.
 10. Inductive heatingdevice according to claim 1, wherein the DC/AC converter comprises aClass-E power amplifier comprising a transistor switch, a transistorswitch driver circuit, and the LC load network configured to operate atlow ohmic load, wherein the LC load network additionally comprises ashunt capacitor.
 11. Inductive heating device according to claim 1,wherein the Class-E power amplifier has an output impedance, and whereinthe power supply electronics further comprises a matching network formatching the output impedance of the Class-E power amplifier to the lowohmic load.
 12. Inductive heating device according to claim 1, whereinthe inductor of the LC load network comprises a helically woundcylindrical inductor coil which is positioned on or adjacent theinternal surface of the cavity.
 13. Aerosol-delivery system comprisingan inductive heating device according to anyone of the preceding claimsand an aerosol-forming substrate comprising a susceptor, wherein atleast a portion of the aerosol-forming substrate to be accommodated inthe cavity of the inductive heating device such that the inductor of theLC load network of the DC/AC converter of the inductive heating deviceis inductively coupled to the susceptor of the aerosol-forming substrateduring operation.
 14. Aerosol-delivery system according to claim 13,wherein the aerosol-forming substrate of the smoking article is atobacco-laden solid aerosol-forming substrate.
 15. Aerosol-deliverysystem according to claim 13, wherein the susceptor is made of stainlesssteel.
 16. Aerosol-delivery system according to claim 15, wherein thesusceptor comprises a flat strip of stainless steel, the flat strip ofstainless steel having a length in a range of about 8 millimeters toabout 15 millimeters, having a width in a range of about 3 millimetersto about 6 millimeters, and having a thickness in a range of about 20micrometers to about 50 micrometers.
 17. Method of operating anaerosol-delivery system according to claim 13, the method comprising thesteps of: determining from the DC supply voltage of the DC power sourceand from the DC current drawn from the DC power source an apparent ohmicresistance, determining from the apparent ohmic resistance thetemperature of the susceptor of the aerosol-forming substrate,monitoring changes in the apparent ohmic resistance and detecting a puffwhen a decrease of the apparent ohmic resistance is determined which isindicative of a temperature decrease of the susceptor during a userinhalation.
 18. Method according to claim 17, wherein the step ofdetecting a puff comprises detecting a puff when the decrease of theapparent ohmic resistance corresponds to a temperature decrease of thesusceptor in the range of 10° C. to 100° C.
 19. Method according toclaim 17, wherein the step of detecting a puff further comprisesallowing for the detection of puffs having a duration in the range of0.5 seconds to 4 seconds.
 20. Method according to claim 17, furthercomprising the steps of counting the puffs already taken from the sameaerosol-forming substrate, and optionally showing to the user the numberof puffs already taken from the same aerosol-forming substrate, or thenumber of puffs remaining to be taken from the same aerosol-formingsubstrate, or both the number of puffs already taken and the number ofpuffs remaining to be taken from the same aerosol-forming substrate. 21.Method according to claim 20, further comprising the step of allowingfor a maximum number of puffs to be taken from the same aerosol-formingsubstrate, and stopping the supply of DC power from the DC power sourceto the DC/AC converter when the counter has counted the maximum numberof puffs taken from the same aerosol-forming substrate.
 22. Methodaccording to claim 17, further comprising the steps of: interrupting thegeneration of AC power by the DC/AC converter when the determinedtemperature of the susceptor of the aerosol-forming substrate is equalto or exceeds a preset threshold temperature, and resuming generation ofAC power when the determined temperature of the susceptor of theaerosol-forming substrate is below the preset threshold temperature(T_(th)) again.